1. Field of the Invention
The present invention relates generally to a zoom lens system, and more particularly to a zoom lens system for use on compact cameras.
2. Description of Related Art
Among zoom lens systems known so far in the art, there are a two-group zoom lens system comprising a first lens group of positive power and a second lens group of negative power, with its focal length varied with a change in the spacing between them, and a three-group zoom lens system comprising a first lens group of positive power, a second lens group of positive power and a third lens group of negative power, wherein the focal length of the system is varied with changes in the spacings between the respective lens groups. The two-group zoom lens system is favorable for cost reductions because it is simpler in lens barrel construction and drive mechanism, and has a smaller number of lenses, than the three-group zoom lens system. To achieve as high a zoom ratio as possible and compactness, however, the three-group zoom lens system is preferable to the two-group zoom lens system. Typical three-group zoom lens systems are disclosed in JP-A's 4-260016 and 5-188296 wherein cost reductions are achieved by minimizing the number of lenses. The former comprises a first lens group consisting of one positive lens, a second lens group consisting of one negative lens and one positive lens and a third lens group consisting of one negative lens, and the latter comprises a first lens group consisting of one negative lens and one positive lens, a second lens group consisting of one negative lens and one positive lens or only one positive lens and a third lens group consisting of one negative lens.
One approach to achieving further cost reductions of zoom lens systems for use on compact cameras is to use plastic lenses. JP-A 4-260016 discloses an example of using plastic lenses for the negative lens of the third lens group and the positive lens in the second lens group. However, the zoom ratio obtained in this example is less than 2 as can be seen from a focal length of about 35 mm to 60 mm. JP-A 5-188296 achieves a zoom ratio of about 2 in an example wherein the second lens group consists of two lenses. Since the lenses of this example are all constructed of glass lenses, however, no satisfactory cost reductions are still achievable.